ATC Altimeter Settings

"Matt Barrow" wrote in message
...

"Frank Ch. Eigler" wrote in message
...

"Matt Barrow" writes:

"Icebound" wrote:
[...]
If the altimeter setting came from a station in the valley 5000 feet
in
true height below the aircraft, the indicated height could differ
from
true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height
from
the aircraft, the difference is likely less than 300 feet. [...]

Mind telling me how that variation (of that magnitude) could come
about?

If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. The effect
is that the lower you are AGL, the closer the calibrated & true
altitudes tend to become, because deviations from the standard
atmosphere become less significant within less tall columns of air.

Ground stations are all AT GROUND LEVEL, whether the station is at 2000
MSL
or 6000 MSL.

Ground stations are all more or less at ground level. But they report
their
altimeter settings in terms of Mean Sea Level, in order to provide a common
basis of comparison. This means that the air pressure read from the
station's
barometer must be *corrected* to sea level. The correction is in terms of
the conditions of the ICAO standard atmosphere, taking account the
difference
in elevation from MSL only: i.e. temperature, humidity etc. or other local
differences in the composition of the atmosphere are not considered in this
correction.

An altimeter can be expected to indicate the correct altitude (+/- 75') only
on the ground at the given ground station, and elsewhere, only where ICAO
standard conditions prevail. For any other position or condition the
altimeter
can be expected to be in error (and the atmosphere can be expected to
always
differ from the ICAO standard conditions).

For positions directly above or below the given station, the amount of
error can be expected to be roughly proportional to the difference between
the actual altitude and the station elevation.

Thus, for a flight at 5000 feet, the altimeter setting for a (nearby) ground
station at 5000 feet elevation can be expected to indicate closer to true
altitude than when using the altimieter setting for a station at sea level.
For a flight nearer sea level, the setting from the (nearby) sea level
station
will indicate closer to the true altitude than the altimeter setting for the
5000'
station.

In either case, there can be expected to be other errors due to the lateral
distance to the station as well, and the difference in atmospheric
conditions
that this entails.

The desire to minimize these effects is why we generally choose the
altimeter
setting from the nearest ground station, or otherwise nearest to our route
of
flight. And why, in the point under discussion, a nearby ground station
located
nearer to our actual altitude is better than one at a different elevation.

"Icebound" wrote in message
...

"Matt Barrow" wrote in message
...


You're bringing in a lot of irrelevant material. The original point was
"Area" altmiter settings and another point made that a station in
mountainous areas would be more accurate if it was located on the
mountain
top at higher altitude than a station at lower altitude in the same
area.

You're still not explaining how a ground station at, say, 6000' MSL
would
be have a more accurate baro reading than one down in a nearby valley
at,
say, 2000' MSL. That is the point of the thread.





There is NO issue of "altimeter-setting accuracy" nor "barometer
accuracy".

Both stations have accurate barometers, and are reading their station
pressure correctly and accurately. Both altimeter-settings are "accurate"
in so far as the settings have been properly determined according to the
rules, for each individual station's actual-air-pressure and each
individual
station's actual-measured-elevation.

But as we all know, setting our altimeter to an "accurate altimeter
setting"
does not mean that the INDICATED altitude matches the TRUE altitude. In
almost never ever does, because the real atmosphere is almost always
different from the "standard" for which altimeters are calibrated.

So there is always a discrepancy between INDICATED and TRUE altitudes. In
most cases this does not matter, because the discrepancy is the same for
everybody.

Somebody in this thread asked WHY this discrepancy was greater if using a
valley station's setting, as opposed to a hill station's setting. (He may
have used the word "accuracy", but his meaning was: "why is my INDICATED
altitude going to be closer to my TRUE altitude when I use the
hill-station's-altimeter setting as opposed to the valley's?")

No, he claimed that ATC uses mountain top reporting stations rather than
lower level ones for "area settings". That makes two fallacies in one post.


My entire tirade was to try to explain why that is so... and just to warn,
that in very cold weather, this means that you are flying much lower (TRUE
altitude) than INDICATED. If you are not paying attention to
indicated-vs-true discrepancies, you may choose an INDICATED altitude
which
may put you below a comfortable margin of terrain/obstacle clearance.

In cold weather, then, it doesn't matter much which reporting station is
used _if pilots don't make necessary corrections for True Altitude_. Same
thing for the other side of the coin, not correcting for density altitude.
Does ATC use valley reporting stations in the summer then?

"Ron McKinnon" wrote in message
news:f8W4e.916805$8l.223757@pd7tw1no...

An altimeter can be expected to indicate the correct altitude (+/- 75')
only
on the ground at the given ground station, and elsewhere, only where ICAO
standard conditions prevail. For any other position or condition the
altimeter
can be expected to be in error (and the atmosphere can be expected to
always
differ from the ICAO standard conditions).

Well, if there is a valley reporting station and nearby mountain top
station, ICAO conditions are not likely to be met in either instance.

The desire to minimize these effects is why we generally choose the
altimeter
setting from the nearest ground station, or otherwise nearest to our route
of
flight.

True, but the point made was that ATC uses higher altitude reporting
stations as "they are more accurate".

And why, in the point under discussion, a nearby ground station located
nearer to our actual altitude is better than one at a different elevation.

Yes, an "area setting" :)

"Matt Barrow" wrote in message
...

In cold weather, then, it doesn't matter much which reporting station is
used _if pilots don't make necessary corrections for True Altitude_.

It matters only because, for the pilot who DID NOT make the necessary
mental adjustment, the discrepancy between indicated and actual altitude
will be GREATER when using the lower-elevation setting, and hence the
possibility of choosing an inappropriate flight altitude (indicate) is also
greater.


Same thing for the other side of the coin, not correcting for density
altitude.
Does ATC use valley reporting stations in the summer then?


It has been repeated in this thread several times:
"The greater the difference in elevation between your aircraft and your
altimeter-setting-station, the GREATER THE DISCREPANCY BETWEEN INDICATED AND
TRUE ALTITUDE."

This is true in ALL cases, cold and hot. In hot weather, the discrepancy
will be in the opposite direction (TRUE altitude will be HIGHER than
INDICATED altitude), but the absolute value of the discrepancy will still be
GREATER when using the valley station.

The way I see it, if you have two stations, one on the ground at 350
feet MSL, and the other on the ground (on top of a mountain) at 5000
feet MSL, and they are both "nearby", then the one that is actually at
5000 feet MSL will give an altimeter setting that will be more accurate
for an airplane that's flying at 5000 feet MSL.

How?

One way to get an altimeter setting is to put an altimeter at the
station, and twist the dial until the actual altitude is shown, and then
read the window. If the station is =at= 5000 feet MSL, then (assuming
horizontal uniformity over the distances involved), any altimeter using
this setting will show an alititude of 5000 feet when they are at the
same altitude as the station. An airplane at 5000 feet using this
setting will show an altitude of 5000 feet.

The station that is physically located at 350 feet MSL would twist the
dial until 350 MSL shows up, and then read the window. IN A STANDARD
ATMOSPHERE, the number in the window should be the same as the one at
the 5000' station. But if the atmosphere is other than standard, the
number in the window (the altimeter setting) will be different. So the
airplane that is using =that= setting, and indicating 5000', will be at
a different altitude from the one using the 5000' station, and also
indicating 5000'. We already know the aircraft using the 5000'
station's setting is dead on, so the other aircraft is off.

Ummmm...air pressure is constant when corrected for altitude. otherwise they
would give altimeter setttings at various altitude, not a various locations.

The correction for altitude presumes a standard atmosphere. When the
atmosphere is nonstandard, this doesn't apply any more.

Of course, the airplane that uses the 5000' station's altimeter setting
while at 5000 feet, and proceeds to land at the 350' high airstrip, will
likely find the indicated altitude once on the ground to be different
from 350' (by about the same amount)

Might you be able to point me to a difinitive paper on that rather than just
idle specualtion?

No.

I keep hearing people running on about that they would be different, but
nothing substantial offered as evidence and certainly nothing I've heard of
in 25 years of flying (but I might have had a deprived career).

Any book on flying, altimeters, and weather, should have a section on
how altimeters guess altitude based on pressure, and the assumptions
that are made in order to do so. Those books will refer to the
"Standard atmosphere", which includes formulas for how air pressure,
temerature, and humidity are presumed to change with altitude. Look up
the "high to low, look out below" rule; it's based on the same idea, and
in the discussion of the origins of this rule of thumb, you should come
across (in print) the same things we are discussing here, and with the
imprimateur of the author of the book.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Matt Barrow" writes:

[...]
The desire to minimize these effects is why we generally choose the
altimeter setting from the nearest ground station, or otherwise
nearest to our route of flight.

True, but the point made was that ATC uses higher altitude reporting
stations as "they are more accurate".
[...]

Matt, you're continuing to pick silly nits instead of showing a
willingness to understand the issue. The higher stations are more
accurate in the clearly stated sense of causing indicated altitudes to
be closer to true altitude. Please stop asking for repeated
elaboration, or attempt historical revision, and just work through
some ground school exercises on the subject.


- FChE

"Jose" wrote in message
om...

The way I see it, if you have two stations, one on the ground at 350 feet
MSL, and the other on the ground (on top of a mountain) at 5000 feet MSL,
and they are both "nearby", then the one that is actually at 5000 feet MSL
will give an altimeter setting that will be more accurate for an airplane
that's flying at 5000 feet MSL.


Unless that airplane is taking off or landing it would seem to be below the
minimum IFR altitude.

The way I see it, if you have two stations, one on the ground at 350 feet
MSL, and the other on the ground (on top of a mountain) at 5000 feet MSL,
and they are both "nearby", then the one that is actually at 5000 feet MSL
will give an altimeter setting that will be more accurate for an airplane
that's flying at 5000 feet MSL.
Unless that airplane is taking off or landing it would seem to be below the
minimum IFR altitude.

.... or separated from the station laterally by a "sufficient" distance,
which will usually not be sufficient for lateral differences in air
pressure to have much effect, but could easily be sufficiently distant
for adequate cumulo-granite separation.

And altimeters are useful for VFR flying too.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Jose" wrote in message
news

... or separated from the station laterally by a "sufficient" distance,
which will usually not be sufficient for lateral differences in air
pressure to have much effect, but could easily be sufficiently distant for
adequate cumulo-granite separation.


The mountain was stated to be "nearby".



And altimeters are useful for VFR flying too.


Yes, but we're discussing this in rec.aviation.ifr.

"Frank Ch. Eigler" wrote in message
...

"Matt Barrow" writes:

[...]
The desire to minimize these effects is why we generally choose the
altimeter setting from the nearest ground station, or otherwise
nearest to our route of flight.

True, but the point made was that ATC uses higher altitude reporting
stations as "they are more accurate".
[...]

Matt, you're continuing to pick silly nits instead of showing a
willingness to understand the issue.

I understand the issue, why don't you stop evading the question?

The higher stations are more
accurate in the clearly stated sense of causing indicated altitudes to
be closer to true altitude. Please stop asking for repeated
elaboration, or attempt historical revision, and just work through
some ground school exercises on the subject.

Now why don't you stop evading the original question/point and point me to
where it says ATC uses high altitude reporting stations because of this
known (I know it too) increase in accuracy?

Someone made a boneheaded assertion and everyone missed (except Newps) that
though they would be more accurate, that ATC does not use such stations for
said accuracy.

So instead of picking nits, why not focus on the question instead of
evading.